A fuel cell system of this type is known from German Patent Application DE 101 25 106 A1. The fuel cell system described there has a preferably electronic switch that allows a switched current flow to be drawn from the fuel cell.
In this connection, the fuel cell is controlled in such a manner that when a critical threshold is reached, such as a minimum voltage, a critical pressure, a predetermined internal resistance, or the like, the switch interrupts the current from the fuel cell in order that the voltage at the individual cells or at one of the individual cells cannot fall below a damaging threshold. Typically, such a damaging threshold is at a cell voltage of 0 volts. While the current flow from the fuel cell is interrupted, the loads are supplied with energy from the electrical energy storage device, here a combination of a battery and a high-performance capacitor. Pulse-width modulated switching, in particular at high frequencies, places very high demands on the switching element itself because the currents generally have to be switched while the system is under load, and because the internal resistance of the switching element should be kept as low as possible to prevent high power dissipation.
These requirements can be achieved by using semiconductor switches, for example in the form of metal-oxide semiconductor based field-effect transistors, so-called MOSFETs. Because these MOSFETs allow extremely short switching times, even under load, it is possible to achieve high pulse-width modulation frequencies combined with moderate power dissipation. In addition, such MOSFETs can cope with extremely high current surges, such as typically occur when using low-resistance intermediate storage devices such as the above-described high-performance capacitor.
In spite of the comparatively moderate power dissipation of such electronic switches, there is still a comparatively large amount of heat generated by the power losses because of the high currents and high powers. In this already very complex fuel cell system, it is a serious disadvantage if special provisions have to be made to dissipate this power loss which manifests itself as thermal energy.
Moreover, especially in fuel cell systems that obtain the hydrogen they need from gas generation systems, the ambient temperature of the electronic switches will be comparatively high, which makes convection cooling with generally used cooling elements or heat sinks even more difficult.